OgreMagi, GPP is pushing KSP to the limit in a variety of ways, which means that it's very particular regarding the way it's installed and exactly which mod dependencies are required.  It's particularly sensitive to versions of other mods, such as Kopernicus and Scatterer.  My advice would be to create a new install somewhere on your computer outside of the Steam directory, then follow the installation instructions a few posts after the OP to the letter.  Lastly, once things are working, never a